1. Field of the Invention
The present invention relates to a high-sensitivity, thin, miniature, electromagnetic polar relay.
2. Description of the Related Art
The cross-sectional views shown in FIGS. 1(a) and 1(b) together with the perspective views shown in FIGS. 1(c) and 1(d) schematically illustrate the structure and operation of a typical electromagnetic miniature polar relay such as disclosed in Japanese Unexamined Patent Publication Toku-Kai-Sho 61-116729. This relay is provided with a coil 1 wound on a bobbin 2, a permanent magnet 6, and an armature 3 which moves due to energization of the coil 1 so as to move contact springs (not shown). The permanent magnet 6 is polarized, for example, as denoted with N and S in FIGS. 1(c) and 1(d). A non-energized state, where no current is applied in the coil 1, is shown in FIGS. 1(a) and 1(c). In this state an end 3a and an end 3b of the armature 3 are moved so as to respectively contact an end 4a of an L-shaped yoke 4 and an end 5a of a U-shaped yoke 5 due to a magnetic flux 6a of the permanent magnet 6. An energized state, where the armature 3 is magnetized due to a current through the coil 1, is shown in FIGS. 1(b) and 1(d). In this state the direction of the current is such that the induced magnetic field is opposite that of the permanent magnet 6. Therefore, the armature end 3a is repelled by the end (N-pole) 4a and is attracted onto an end (S-pole) 5b of the U-shaped yoke 5, and the other armature end 3b is magnetically attracted to contact the other end 5a of a U-shaped yoke 5, due to a magnetic flux 1a of the coil as shown in FIG. 1( d). In this state the armature end 3b and the end 5a of the U-shaped yoke 5 tend to repel each other; however, they are kept in contact by a leaf spring 7. One end of leaf spring 7 is fixed to the armature 3 as seen in FIGS. 1(a) and 1(b). After the armature position is switched, the end 3b of the armature 3 and the end 5a of the yoke 5 are magnetically attracted to each other, and thus contact each other.
Operational characteristics of the FIG. 1 relay are shown in FIG. 2, where the abscissa indicates armature position on its stroke, and the ordinate indicates mechanical force on the armature. In FIG. 2, curve A denotes a load characteristics of the contact spring. That is, curve A represents a mechanical load on the armature during the armature stroke, and more particularly the force tending to push the armature back to the center. This mechanical load is zero at the center of the stroke, and gradually increases as the armature deviates from the center of the stroke due to bending of a contact spring. At kink points K and K' of curve A, a contact on the contact spring begins to touch a stationary contact. Further deviation of the armature towards a magnetic pole 4a or 5b causes further bending of the contact spring. As indicated by FIG. 2, this further bending requires a layer force.
In FIG. 2, curve B denotes a mechanical force magnetically induced on the armature by the permanent magnet 6. Curve B is shown as a negative force. This means that the force is towards N-pole 4a. Curve B must be always below the curve A. The gap between the curves A and B is a margin for variation of various conditions. At the N-Pole 4a, the difference F.sub.B between the holding force Fgr and the load P.sub.B indicates a pressure on the contacts, and is a margin that protects tho contacts from external shock or chattering.
A curve C denotes a mechanical force magnetically induced on the armature as a sum of magnetic forces of the permanent magnet 6 and the energized coil 1, to which the current is applied. The direction of this force is opposite that of the magnetic field of the permanent magnet 6. Curve C is shown as a positive force. This means that the force is towards S-pole 5b. Curve C must be always above the curve A. When armature 3 is at the S-pole 5b, the difference between the holding force Pgr and the mechanical load P.sub.B ' indicates a pressure on the stationary contacts and protects the contacts from external shock or chattering.
In an electromagnetic polar relay having structure as described above, the desirable characteristics for achieving a high sensitivity, i.e. low coil energization power, and reliable performance are as follows: Curves B and C must have enough margin (e.q., F.sub.B ', F.sub.8) with respect to curve A. However, the margin should not be too much, i.e., should be as small as possible. This is because the margin of curve C to curve A requires excessive ampere-turns, i.e. coil power consumption. However, because of magnetic characteristics of some permanent magnet materials the value of curve B (i.e. F.sub.B) becomes very large at the N-pole. In order to overcome this large value, the coil requires large ampere-turns which causes high power consumption and a very excessive margin at the S-pole.